I do my magic in three different languages, but I have never noticed any difference.
Spells backfire for many different reasons, but not because one used Spanish instead of Latin. The only way it could make a difference, when you really believe it. Like believing that after doing a spell, you have to forget it. I don't forget my spell, I don't cast them in Latin - and it works for me very well.
If I were you - I would check every thing he says just to be safe.